Earth-boring bits of the type concerned herein have a bit body with three depending bit legs. A rolling cutter is rotatably mounted to each bit leg. Each cutter has a plurality of rows of milled teeth formed in the cutter shell by machining. The teeth are hardfaced with hardfacing that is typically tungsten carbide in an alloy steel matrix. One of the cutters has what is referred to as a spear point.
The spear point protrudes from an apex of the cutter shell to the vicinity of the longitudinal axis of the bit. This spear point has a neck which joins a conical portion of the cutter shell. A plurality of blades extend from the neck and converge to an apex. Hardfacing is applied to the blades, the hardfacing being the same type as used on the teeth. The blades have valleys or spaces between them. The neck and the spaces between are free of any hardfacing.
Bits of this type often have a center nozzle as well as lateral nozzles. The center nozzle is located on the longitudinal axis of the bit. The lateral nozzles are located at the sides of the bit body between two of the legs. The nozzles communicate with a central passage in the bit body for discharging drilling mud. The high pressure jets being discharged through the nozzles circulate cuttings back to the surface through an annulus surrounding the drill string. The drilling fluid being discharged through the center jet directly impinges on the spear point. The drilling fluid is abrasive and causes erosion to the cutter shell, eroding the neck and the valleys of the spear point. Even though the blades are hardfaced, eventually the spear point may break off. Once broken off, it is likely to interfere with the teeth of the cutters, causing tooth breakage. This leads to premature failure of the bit.
Another type of earth-boring bit uses tungsten carbide inserts pressed into mating holes formed in the cutter shell. These insert bits typically do not utilize a center jet. Insert bits of this nature have also been used in mining operations. In a mining operation, air is discharged through the nozzles, rather than drilling fluid. Air is normally not considered abrasive, unlike drilling mud. In one instance, hardfacing was placed on the spear point area surrounding TCI inserts of a mining bit to avoid erosion to the shell due to contact with the earth formation.